Neurophysiology Part 18 - The Vestibular System Fall 2024 PDF

Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: Describe the location and components of the vestibular system Describe the functions of the Vestibular System Understand th...

Andre Azevedo, DVM, MSc Assistant Professor of Veterinary Physiology [email protected]  At the end of the lecture, students should be able to: Describe the location and components of the vestibular system Describe the functions of the Vestibular System Understand the major clinical signs of peripheral vestibular disease Understand how to differentiate peripheral from central vestibular disease  Divided into peripheral and central Functions: maintain balance and posture, coordination of eye-head-body movement PERIPHERAL CENTRAL (outside the brain) (cerebellum and brainstem) Vestibular apparatus in membranous Four pairs of vestibular nuclei on each side in labyrinth the brainstem The vestibular portion of the Cerebellum (fastigial nucleus and vestibulocochlear nerve (CN VIII) flocculonodular lobe) in  inner ear Baesthum THE MEMBRANOUS LABIRINTH The vestibular apparatus is a bilateral receptor system located in the membranous labyrinth: semicircular canals IT IS A SERIES OF FLUID-FILED STRUCTURES + otolith organs 3 SEMICIRCULAR CANALS Kinetic labyrinth Rotation detection 2 OTOLITH ORGANS Utricle and Saccule Greek Oto = ear ; Lithos = Stone Static labyrinth linear acceleration detection COCHLEA Audition detect  P THE SEMI-CIRCULAR CANALS Filled with endolymph and 3 on eachside ofhead surrounded by perilymph Anterior (superior) canal Lateral (horizontal) canal Posterior canal Named according to its position in space Contain the hair cells THE VESTIBULAR HAIR CELLS halells Have a large kinocilium Tonically active cells at resting state Constant firing rate when not stimulated If the stereocilia are bent toward one side, troeside the AP frequency increases lithiffactory The opposite makes the frequency decrease The change in frequency is how the brain knows for which side we are moving  THE VESTIBULAR HAIR CELLS - CANALS Located in THE CRISTA AMPULLARIS of the semicircular canals and covered by a gelatinous mass called CUPULA During angular acceleration of the head, the cupula is displaced (inertial force), causing excitation or inhibition of the hair cells  cilia here bathe thymine As the head moves, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. All six canals activation/deactivation can give a very precise indication of head movement in three dimensions. IF WE SPIN AROUND AND SUDDENLY STOP, WHY DO WE FEEL DIZZY? we when move endolymph takes abruptly stop endolymph  a moment to stop THE OTOLITH ORGANS UTRICLE AND SACCULE detect linear acceleration and deceleration (inertial force) and static tilt of the head (gravitational force) Hair cells are located in the macula of the otolith organs  THE VESTIBULAR HAIR CELLS - OTOLITHS Located in the MACULA of the otolith HORIZONTALLY ORIENTED VERTICALLY ORIENTED organs. An otolith mass overlies the vestibular hair cells. When the head is tilted, gravitational forces act on the otolith mass, moving it across the vestibular hair cells During linear acceleration, inertial forces act on the otolith mass, also moving it across the vestibular hair cells The cells are either activated or inhibited to alert about a change in the position of the head  forswati HORIZONTALLY VERTICALLY duduetto I ORIENTED ORIENTED movement HORIZONTALLY VERTICALLY ORIENTED ORIENTED  THE MACULAE HAVE HAIR CELLS ORIENTED IN DIFFERENT DIRECTIONS TO GET INFORMATION ABOUT LINEAR MOVEMENT IN DIFFERENT ANGLES. 0 360 270 90 180  The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head or if the head starts moving in a straight line. Gravitational or inertial forces over the otolith causes the stereocilia to bend in the direction of that linear acceleration. DOES IT HELP TO CLOSE YOUR EYES DURING A ROLLER COASTER RIDE? A.Yes B. No VESTIBULAR PATHWAY Sensory vestibular fibers arrive to: 4 vestibular nuclei Cerebellum fastigial nucleus and flocculonodular lobe via caudal cerebellar peduncle Vestibular nuclei project to: Spinal cord lateral and medial vestibulospinal tracts Brainstem www.anifiese mon motor nuclei of CN3, CN4 and CN6 eye reticular formation  vomiting center vomitinduced bydizzy Cerebellum toxins fastigial nucleus and flocculonodular lobe via caudal cerebellar peduncle Thalamus relay info to cortex for conscious vestibular information by received temporal lobe cortex End B1 8 VESTIBULAR DYSFUNCTION Just a cool piece of medieval art for your appreciation. CLINICAL SIGNS CLINICAL SIGNS CLINICAL SIGNS – NOT VESTIBULAR! cortex problem no lost balance  Detect movement of head As a sensor of gravity and head acceleration, the vestibular system is one of the most important tools in controlling posture In addition to providing sensory information, the vestibular system also contributes directly to motor control Descending motor pathways such as the vestibulospinal tracts receive vestibular and other types of information to control eye, head, and trunk orientation and to coordinate postural movements When all the components of the vestibular Extensor mm. system are working, there is equal extensor tone on each side and balance, and exterpine equilibrium are maintained and adjusted With a lesion on one side, there will be a lack of facilitation of the extensor muscles on Normal that side, leading to imbalance. The normal side will “push” the body and head toward the abnormal side This is manifested clinically as head tilt, circling, leaning, falling, or rolling to the side of the lesion. Right lesion Head tilt to the right  This is manifested clinically as head tilt, circling, leaning, falling, or rolling to the side of the lesion. A severe presentation of the vestibular signs does NOT mean poor prognosis. CLINICAL vertical us SIGNS nystagm phase w fast dorsal iii ftp.fx w  nystagmus is a rhythmic, involuntary oscillation of the eyes pendular jerk nystagmus nystagmus have equal have fast and movements slow phase (fast phase away from the side of the lesion) not a sign of vestibular disease horizontal, rotatory or vertical ex: Siamese – benign conjugate (both eyes moving congenital defect in in the same direction) or not the visual pathway spontaneous or positional  The vestibular system is also responsible for maintaining the position of the eyes relative to the position of the head in space Medial longitudinal fasciculus Control the extraocular muscles of the eye VESTIBULOOCULAR REFLEX Coordinates eye and head movements so that the head turns, and the eyes remain fixed on the original field of vision for as long as possible The eyes slowly move opposite head rotation followed Allows the animal to interpret a field of vision despite by a fast flick back in the direction of head rotation = angular acceleration of the head physiological nystagmus This reflex eye movement pattern requires normal sensory input from semicircular ducts, an intact pathway in the brainstem, and normal function of motor neurons and extraocular muscles  not receiving right imput from vesting stem Spontaneous nystagmus is caused by abnormal, asymmetric action potential inputs to the brainstem from the vestibular apparatus on the 2 sides of the head The imbalance in neural activity is interpreted by the brainstem as a rotation or movement of the body. Nystagmus is generated even though the body and head are stationary  Animals with vestibular dysfunction will usually present with a primary complaint of head tilt, VESTIBULAR DYSFUNCTION nystagmus or vestibular ataxia (or combination of all three) CENTRAL VESTIBULAR DYSFUNCTION For central disease, look for brainstem or cerebellum additional brainstem or cerebellar signs PERIPHERAL VESTIBULAR DYSFUNCTION inner ear (vestibular apparatus) or CN VIII CLINICAL SIGN PERIPHERAL CENTRAL Head tilt Yes Yes Ataxia, falling, rolling Yes Yes Clinical signs Strabismus Yes Yes Head tilt Nystagmus Horizontal, rotatory Horizontal, rotatory, vertical Vestibular ataxia Altered mentation No Possible Falling/rolling These signs can be CENTRAL Proprioceptive No Usually OR PERIPHERAL deficits Nystagmus vertical (except for vertical nystagmus CN deficits Only VII farial Possible Strabismus - always central) Horner’s syndrome Possible Rarely Cerebellar signs No Possible VESTIBULAR DYSFUNCTION “VESTIBULAR DISEASE” IS NOT A DIAGNOSIS! IT IS A GROUP OF CLINICAL SIGNS SHOWING THE VESTIBULAR SYSTEM IS NOT WORKING PROPERLY CAN BE CAUSED BY ANY DISEASE AFFECTING PERYPHERAL OR CENTRAL VESTIBULAR COMPONENTS PRIMARY DISEASE MUST BE INVESTIGATED BETTER TERMS TO REFER TO THE CONDITION UNTIL DIAGNOSIS WOULD BE “VESTIBULAR DYSFUNCTION” OR “VESTIBULAR SYNDROME”  Common cause for vestibular dysfunction in dogs and cats Clinical signs are often severe, with rolling and rapid nystagmus quite evident ALWAYS PERIPHERAL SIGNS ONLY  There is no such thing as CENTRAL IDIOPATHIC VESTIBULAR DISEASE There is no sex or breed predilection Cats: the average age is 4 years, but any age can be affected Dogs: usually senior/geriatric dogs Dr. Lemos Good prognosis - signs resolve rapidly without definitive treatment 1 -3 weeks Diagnosis is through excluding all other causes of peripheral vestibular dysfunction (diagnosis of exclusion) Ex: Otitis Some animals require fluid and antiemetic therapy Dr. Vieira Dr. Vieira